This invention relates to electric discharge machining apparatuses with a wire-shaped electrode for machining a workpiece by the use of electric discharge, and more particularly to a control device for use in the electric discharge machining apparatus, which is capable of optionally controlling machining electrical conditions according to variations in thickness of the workpiece and to the quality thereof.
In an electric discharge machining apparatus of this type, a metal wire about 0.05 to 0.3 mm in diameter is employed as its one electrode. Electrical discharge is caused between the electrode and a workpiece to be machined to melt the corresponding portion of the workpiece. The electrode is displaced relative to the workpiece in the X-Y plane to thereby electrically cut or cut a way the workpiece in a desired configuration. In this case, in general, the electrical discharge is carried out with a constant speed feed of 1 pulse/.mu.m; that is, the discharge is effected at a constant voltage in the machining gap without controlling the discharge energy.
This constant speed feed method can satisfactorily machine a workpiece if the workpiece is constant in thickness. However, in machining a workpiece which is variable in thickness, initially it is necessary to set the machining feed speed to a value for the maximum thickness (that is, the maximum machining area) of the workpiece, so that no short-circuiting occurs and the wire-shaped electrode may not be broken. Accordingly, in the constant speed feed method, the machining feed speed is relatively low. Thus, even in machining a portion of the workpiece which is smaller in thickness than the other portions, the machining feed speed is still maintained low. Therefore, the constant speed feed method is considerably low in machining efficiency. On the other hand, it is well known in the art that even if a workpiece is constant in thickness, in order to shape a workpiece to a configuration having an angular corner it is preferable to increase the machining feed speed or to decrease the discharge energy. Because the workpiece is machined sharply, i.e., the resultant corner is not round. Thus, the constant speed feed method in which the discharge energy is maintained unchanged still involves problems to be solved.
An electrical discharge machining apparatus has been proposed in the art in order to eliminate the above-described difficulties accompanying the constant speed machining method. In the conventional apparatus, a voltage across a machining gap, i.e., a machining voltage, is detected and a machining feed speed is controlled so that the voltage thus detected is constant. The arrangement and operation of the conventional apparatus will be described with reference to FIG. 1.
A machining electric source (3) supplies a machining current to a wire electrode (1) and a workpiece (2) to be machined. The average value Eg of the machining voltage and a reference voltage Eo are applied to an error voltage amplifier (4), which determines a machining feed speed F proportional to an error voltage which is the difference value between the machining voltage Eg and the reference voltage Eo. The machining feed speed F thus determined is distributed as an X-axis component Fx and a Y-axis component Fy by a speed distributor (5), which are adapted to drive an X-axis motor (6) and a Y-axis motor (7), respectively. In this connection, the following relation is established between the machining feed speed F provided by the amplifier (4) and the outputs Fx and Fy of the speed distributor (5): EQU Fx.sup.2 +Fy.sup.2 =F.sup.2
In the above-described arrangement, when the gap between the wire-shaped electrode (1) and the workpiece (2) becomes smaller and accordingly the machining voltage Eg becomes lower than the reference voltage Eo, the machining feed speed F is reduced and the gap is widened so that the machining voltage Eg approaches the reference voltage Eo. In contrast, when the machining voltage Eg becomes higher than the reference voltage Eo, the machining feed speed F is increased to thereby cause the machining voltage Eg to approach the reference voltage Eo. This is a system in which a machining voltage is fed back to vary the machining feed speed. In this system, the machining feed speed is increased when a relatively thin portion of the workpiece is being machined, whereas it is decreased when a relatively thick portion of the workpiece is being machined.
The employment of the above-described machining feed speed control in which the machining voltage is maintained unchanged makes it possible to eliminate to some extent the loss in machining feed speed which is involved in constant speed feed.
FIG. 2 shows the waveform of current for charging a charge-discharge capacitor. In FIG. 2, Ip is the peak value of the charging rest current, .tau.p is the pulse width, and .tau.r is the pause period of time. FIG. 3 is a circuit diagram showing the machining electric source (3) in FIG. 1. The electric source (3), as shown in FIG. 3, comprises a capacitor (8) which affects a machined surface roughness, a current limiting resistor (9) determining the peak value Ip of the charging current, a switching transistor (10), an oscillator (11) for determining the pulse width .tau.p and the rest period of time .tau.r of the machining current, and an internal DC source (12) providing a no-load voltage between the electrodes.
Depending on these electrical conditions, discharge machining energy is variable even with the average machining voltage Eg maintained unchanged. In general, in machining a workpiece relatively small in thickness the discharge machining energy is liable to concentrate at a point, and therefore it is necessary to lower the above-described electrical conditions to decrease the discharge machining energy, otherwise the wire electrode would be broken.
The machining feed speed F is so controlled that the machining voltage Eg is maintained unchanged and in machining a workpiece variable in thickness the electrical conditions are so set that the wire electrode (1) is not broken at the portion of the workpiece which is the smallest in thickness. Therefore, at the portion of the workpiece which is larger in thickness, the electrical conditions are not sufficient and accordingly the machining speed is decreased. In addition, it is known that the machining accuracy is improved by increasing the electrical conditions at the relatively large portion of the workpiece in thickness.
When the machining direction is changed as in the corner of a machining configuration, the discharging area is decreased, i.e., equivalently the thickness is decreased. Therefore, with the electrical conditions maintained unchanged, over-cutting occurs, as a result of which the accuracy of the machined corner is lowered. Accordingly, in this case, it is necessary to decrease the electrical conditions in order to machine the workpiece sharply at the corner with high accuracy.
As is apparent from the above description, the conventional constant speed feed method and the conventional machining speed control maintaining a machining voltage unchanged are disadvantageous in that in machining a workpiece variable in of variable discharge machining area, i.e., in thickness or in machining a workpiece for providing corners, the machining feed speed and the machining accuracy are unsatisfactory. Furthermore, the conventional methods are low in reliability because the electrical conditions are manually set, with the result that the machining operation depends greatly on the operator's experience and because the setting of the electrical conditions is rather difficult, with the result that the wire-shaped electrode is often broken.